Self-constrained anisotropic germanium nanostructure from electroplating

ABSTRACT

A nanostructure comprising germanium, including wires of less than 1 micron in diameter and walls of less than 1 micron in width, in contact with the substrate and extending outward from the substrate is provided along with a method of preparation.

CROSS REFERENCE TO RELATED APPLICATION

The present application is related to co-pending U.S. patent applicationentitled “Structures Containing Electrodeposited Germanium And MethodsFor Their Fabrication” Ser. No. 11/620,361, which is filed on even dateherewith and is assigned to International Business Machines Corporation,the assignee of the present application. The entire contents of suchco-pending U.S. patent application are incorporated herein by referencefor all purposes.

TECHNICAL FIELD

The field of the present disclosure relates to the nanostructurecomprising germanium (Ge) nanowires. In particular, the presentdisclosure relates to electrodeposition of Ge onto patternedsemiconductor substrates.

BACKGROUND

Nanowire field effect transistors would allow 3-dimensional integrationand thus a much higher device density than the current technologies. Thekey element of such a device is the semiconductor nanowire. So far,nanowires of a few semiconductor compounds produced by electroplatinghave been reported. However, nanowires of Si and Ge have only beenproduced by vacuum processes.

Moreover the previously reported nanowire structures made fromelectroplating are produced with a template, which defines the shape ofthe structures. As described in FIG. 1, the template (100) withnanopores (101) has a layer of conductive material (102) at one side,which carries the electroplating current and wire structures (103) areelectroplated with the shape defined by the pores.

Germanium is a semiconducting material with higher mobility as comparedto silicon. There have been some limited suggestions ofelectrodeposition of germanium onto metals. However, they have not beenespecially successful. These efforts may have not been thwarted due tothe high reversible potential of Ge and the very low hydrogenoverpotential on Ge surfaces. Therefore, all of the plating current canresult from the proton reduction (side reaction) and no Ge plating canoccur once the electrode surface is covered by Ge. So far, threeapproaches of Ge electroplating have been reported.

For instance, plating of germanium on metal substrates has been reportedin alkaline aqueous solutions (see Fink et al., Journal of theElectrochemical Society, vol. 95, p. 80 (1948)) and in glycol solutions(see Szekely, Journal of the Electrochemical Society, vol. 98, p. 318(1951) and U.S. Pat. No. 2,690,422 to Szekely). More recently, somestudies have been reported directed nucleation studies in ionic liquidmedia. See Endres, Electrochemical and Solid State Letters, vol. 5, p.C38 (2002); Endres, Physical Chemistry and Chemical Physics, vol. 4, p.1640 (2002) and Endres, Physical Chemistry and Chemical Physics, vol. 4,p. 1649 (2002).

In the aqueous solution approach, an extremely alkaline solution (pH>13)was suggested to minimize the proton concentration and thereforesuppress the reduction of proton. This approach is not compatible withmost microelectronics processes due to the extremely high pH of theelectrolyte, which cause damages to most of the structures that arebuilt from dielectrics such as silicon oxides.

In the non-aqueous solution approaches, where the glycol and ionicliquid solvents are used, the proton is either partially decreased orcompletely removed in the solution resulting in a much less sidereaction. The techniques using ionic liquids suffer from thedisadvantages of high viscosity and expenses.

Electroplating of germaniun on semiconductor substrates and especiallyon silicon has not been previously achieved, except as disclosed theco-pending U.S. patent application entitled “Structures ContainingElectrodeposited Germanium And Methods For Their Fabrication” (AttorneyDocket No. YOR9-20060444US1), which is assigned to the same assignee andwhich discloses a method to produce epitaxial Ge thin film onsemiconductor substrates by electroplating and solid state epitaxy. Insaid patent application, a method to electroplate free standing Genano-structure without being constrained in a template is disclosed.

SUMMARY

The present disclosure relates to a nanostructure that comprises asemiconductor substrate, and comprises Ge in contact with the substrate.The Ge is less than 1 micron in at least one dimension. In oneembodiment the Ge extends outwardly from its substrate.

The present disclosure also relates to a method for forming aGe-containing nanostructure that comprises:

-   -   obtaining a patterned semiconductor substrate,    -   immersing the semiconductor substrate into a plating solution        containing Ge,    -   applying electrical potential between the semiconductor        substrate and an anode in the plating solution whereby the        patterned semiconductor substrate, wherein Ge is plated through        the openings in the pattern.

The Ge is less than 1 micron in at least one dimension. In oneembodiment the Ge extends outwardly from its substrate and the Ge isless than 1 micron in diameter.

Still other objects and advantages of the present disclosure will becomereadily apparent by those skilled in the art from the following detaileddescription, wherein it is shown and described only in the preferredembodiments, simply by way of illustration of the best mode. As will berealized, the disclosure is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, without departing from the intent of this disclosure.Accordingly, the description is to be regarded as illustrative in natureand not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams of prior art in electroplatingmetallic nanostructures.

FIGS. 2A and 2B are schematic diagrams of prior art of a mushroom growthas is in most cases of electroplating through mask.

FIG. 2C is a schematic diagram of a nanostructure according to thisinvention.

FIG. 3A is an electron microscope photograph of a patternedsemiconductor substrate prior to plating according to the presentdisclosure.

FIGS. 3B and 3C are electron microscope photographs at differentmagnifications illustrating the germanium nanowires grown out of thepatterned substrate of FIG. 3A.

FIG. 4 is an electron microscope photograph illustrating germanium wiresgrown out of a patterned semiconductor substrate.

FIG. 5 shows Ge XPS analysis of a germanium nanowire electroplatedaccording to the present disclosure. The two peaks are denoted aselemented germanium and germanium oxide.

FIGS. 6A and 6B are electron microscope photographs illustratinggermanium wire- and wall-shaped structures grown out of a patternedsemiconductor substrate.

FIG. 7 shows a schematic representation of an exemplary apparatus forelectrodepositing Ge on semiconductor wafer substrates.

BEST AND VARIOUS MODES FOR CARRYING OUT DISCLOSURE

The present disclosure relates to providing germanium nanostructures onsemiconductor substrates. Suitable semiconductor conductor substratesinclude but not limited to: Si-based and Ge-based semiconductors such asSi, Ge, SiGe alloys, SiC alloys, SiGeC alloys as well as III-V and II-VIsemiconductors as well as any combinations of the above.

According to the present disclosure germanium nanostructures areelectroplated onto patterned semiconductor substrates having insulatingregions and semiconductor regions. The insulating regions can be formedfrom dielectrics such as silicon oxides, silicon nitrides and aluminumoxide.

The germanium can be electroplated directly onto the semiconductorsubstrate without any seed or intermediate layer such as a seed layer.If desired a seed layer such as copper or nickel layer can be depositedprior to the electroplating of the germanium. This seed layer can beformed prior to the fabrication of the dielectric stencil by standarddeposition processes such as sputtering, evaporation, PVD and CVD. Whenpresent, the seed layer is typically about 10 to about 100 nanometersthick and more typically about 10 to about 20 nanometers thick. Otherintermediate layer can also be deposited prior to the germanium platingby electrochemical deposition including electroless or electroplatingtechniques.

The electroplating bath typically employed according to the presentdisclosure contains a source of germanium ions such as germaniumtetrahalide and especially germanium tetrachloride and an organicsolvent such as an alkyl diol. Examples of alkyl diols are alkyl diolstypically having 2-5 carbon atoms and including ethylene glycol,propylene glycol, 1,3 propanediol, butylene glycol, 1,3 butanediol andpentylene glycol. The concentration of the source of germanium istypically about 0.2 to about 0.7 moles and more typically about 0.3 toabout 0.5 moles/liter; an example being about 0.5 mol/L GeCl₄. Thecomponents of the electroplating bath are desirably highly pure(>99.9%).

FIG. 7 shows a plating apparatus 10 that may be used to implement the Geelectroplating of the present disclosure. Apparatus 10 comprises vessel20 containing electrolyte 30 and stationary anode 40. Substrate 50 ismounted on rotating cathode 60 and immersed in electrolyte 30. Rotatingcathode 60 and edges of substrate 50 are protected from the electrolyteby insulating housing 70. Power supply 80 supplies a current betweenanode 40 and substrate/cathode 50/60. A typical anode 40 is graphite.

The electrodeposition is typically carried out at temperature of about50° C. to about 90° C. and more typically at about 70 to 80° C. Thecurrent density employed is typically about 2000 to about 8000milliamps/cm². Prior to immersing the semiconductor substrate into theplating bath, the substrate is typically cleaned, such as by washingwith a dilute HF solution (1:100). The plating both is typically agedsuch as for at least about 3 hours or in the alternative contains asmall amount of water, e.g. about 0.5 g/L to about 2 g/L, a typicalexample being 1 g/L.

During plating the cathode rotates at a typical rate of about 500 rpm toabout 2000 rpm with an example being about 1000 rpm. The electroplatingis carried in a galvanostatic mode. The plating is carried out forsufficient time to provide germanium extending outwardly from thesubstrate typically to a length of about 0.1 μm to about 3 μm, moretypically about 0.2 μm to about 1 μm.

As mentioned above, the present disclosure makes it possible tofabricate germanium nanowire structures without the use of template. Themethod of the present disclosure makes it possible to produce germaniumnanowires by a self-constrained anistropic growth in the electroplating,completely different from a traditional template plating process. Asdescribed in FIG. 2, shallow patterns (200) with pores (201) arefabricated on a conductive or semi-conductive layer (202). Platingcontinues after the pores are filled. Instead of a mushroom cap (203)that occurs in most cases of electroplating, wire structures (204) areproduced as they grow out of the pattern in an anisotropic mannermaintaining the size defined by the pattern.

The following non-limiting examples are presented to further illustratethe present disclosure.

EXAMPLE 1

The electrolyte employed comprises a 0.5 mol/L solution of GeCl₄ in1,3-propanediol. Both chemicals are highly pure and dry. The cathode isa patterned silicon having been pre-cleaned and patterned by standardmicroelectronics fabrication processes. A typical stack for patterningis prepared by depositing on silicon, about 10 nanometers thick ofsilicon dioxide and then, 20 nanometers thick silicon nitride. Featureswith 200 nanometers in dimension are patterned through silicon nitrideand silicon dioxide with lithography and reactive ion etch processes.Immediately before the electroplating, the patterned silicon substrateis dipped in a 1:100 dilute HF for about 2 minutes to remove nativeoxide. InGa eutectic is applied on the backside of the silicon to forman ohmic contact, and the substrate is mounted on a rotating diskelectrode as illustrated in FIG. 7.

The fresh GeCl₄ bath is aged by exposing to ambient conditions for atleast 3 hours. The electrolyte is maintained at about 70-80° C. Agraphite anode is used. The cathode rotates during plating at a typicalrate of 1000 rpm. The electroplating is carried out in a galvanostaticmode and a current of about −4000 mA/cm2 is typically used for producingthe nanostructure.

FIGS. 3A and 3C show a typical example of the wire structure plated at atypical condition according to this example. FIG. 3A shows the patternbefore plating, a stack (300) 10 nanometers silicon dioxide and then, 20nanometers silicon nitride are deposited on the silicon. Vias of 200nanometers diameter (301) are patterned with lithography and etchingprocesses and the silicon substrate is exposed. The electroplatedgermanium nanowire structures (302) grow out of the pattern and maintainthe diameter of the vias, 200 nm. (See FIGS. 3B-3C). FIG. 4 is a wirestructure grown at the same conditions as FIG. 3, but for a longer time.Miconmeter-long wires are obtained with the shape still defined by thepattern.

FIG. 6 shows the XPS analysis of a typical wire structure electroplatedaccording to this example. Two peaks are denoted as elemental germaniumand germanium oxide. The majority of the deposit is confirmed asgermanium while the surface of the structure can be oxidized in the air.

EXAMPLE 2

The electrolyte employed comprises a 0.5 mol/L solution of GeCl₄ in1,3-propanediol. Both chemicals are highly pure and dry. The cathode isa patterned silicon having been pre-cleaned and patterned by standardmicroelectronics fabrication processes. A typical stack for patterningis prepared by depositing on silicon, about 10 nanometers thick ofsilicon dioxide and then, 20 nanometers thick silicon nitride. Featureswith 200 nanometers in dimension are patterned through silicon nitrideand silicon dioxide with lithography and reactive ion etch processes.Immediately before the electroplating, the patterned silicon substrateis dipped in a 1:100 dilute HF for about 2 minutes to remove nativeoxide. InGa eutectic is applied on the backside of the silicon to forman ohmic contact, and the substrate is mounted on a rotating diskelectrode.

About 1 g/L of water is added to the fresh GeCl₄ bath. The electrolyteis maintained at about 70-80° C. A graphite anode is used. The cathoderotates during plating at a typical rate of 1000 rpm. The electroplatingis carried out in a galvanostatic mode and a current of about −4000mA/cm2 is typically used for producing the nanostructure.

FIGS. 7A and 7B show typical structures plated at a typical conditionsaccording to this example, on via and stripe patterns. FIG. 7A showsthat the electroplated germanium nanowire structures grow out of the viapatterns and maintain the diameter of the vias, 200 nanometers. FIG. 7Bis a wall structure grown at the same conditions in this example, but ona stripe pattern shown in FIG. 4B. The wall structures grow out of thestripe patterns and maintain the shape of the stripes, hundreds ofmicrometers in length and 200 nanometers in width.

The term “comprising” (and its grammatical variations) as used herein isused in the inclusive sense of “having” or “including” and not in theexclusive sense of “consisting only of”. The terms “a” and “the” as usedherein are understood to encompass the plural as well as the singular.

The foregoing description illustrates and describes the presentdisclosure. Additionally, the disclosure shows and describes only thepreferred embodiments of the disclosure, but, as mentioned above, it isto be understood that it is capable of changes or modifications withinthe scope of the concept as expressed herein, commensurate with theabove teachings and/or skill or knowledge of the relevant art. Theembodiments described hereinabove are further intended to explain bestmodes known of practicing the invention and to enable others skilled inthe art to utilize the disclosure in such, or other, embodiments andwith the various modifications required by the particular applicationsor used disclosed herein. Accordingly, the description is not intendedto limit the invention to the form disclosed herein. Also, it isintended that the appended claims be constructed to include alternativeembodiments.

All publications, patents and patent applications cited in thisspecification are herein incorporated by reference, and for any and allpurposed, as if each individual publication, patent or patentapplication were specifically and individually indicates to beincorporated by reference. In the case of inconsistencies, the presentdisclosure will prevail.

1. A method for forming a germanium-containing nanostructure comprising:forming a mask on a semiconductor substrate, wherein said mask has atleast one opening to expose said substrate; immersing said semiconductorsubstrate with said mask into a plating solution containing Ge; applyingelectric power between said semiconductor substrate and an anode in saidplating bath whereby said semiconductor substrate where exposed isplated with germanium; wherein said germanium is plated directly on saidsemiconductor substrate without a seed layer on the semiconductorsubstrate; and wherein said germanium extends outwardly from saidopening in contact with said substrate and any cross section of saidgermanium taken parallel with said substrate mimics the shape of saidopening of the said mask.
 2. The method of claim 1 wherein saidgermanium-containing nanostructure comprises nanowires with a diameterof about 100 to about 300 nanometers.
 3. The method of claim 1 whereinsaid germanium-containing nanostructure comprises wall shaped structureswith a width of about 100 to about 300 nanometers.
 4. The method ofclaim 1 wherein said semiconductor substrate is selected from the groupconsisting of Si, Ge, SiGe, SiC, GeC, SiGeC, III-V materials, and II-VImaterials and mixtures thereof.
 5. The method of claim 1 wherein saidsubstrate is selected from the group consisting of Si, Ge and GaAs. 6.The method of claim 1 wherein said substrate contains dielectricregions.
 7. The method of claim 1 wherein the plating bath contains analkyl diol solvent.
 8. The method of claim 6 wherein the alkyl diolsolvent is selected from the group consisting of ethanediol,propanediol, butanediol and pentanediol.
 9. The method of claim 8wherein the alkyl diol is selected from the group consisting of ethyleneglycol, propylene glycol and 1,3-propanediol.
 10. The method of claim 6wherein the current density of the electroplating is about −2000 toabout −8000 milliamps/cm².
 11. The method of claim 6 wherein the bath isaged for about 3 to about 6 hours prior to the electroplating.
 12. Themethod of claim 6 wherein the source of germanium comprise GeCl₄. 13.The method of claim 6 wherein the bath further comprises about 0.5 g/Lto about 2 g/L by weight of water.
 14. The method of claim 1 whereinsaid germanium grows by a self-constrained anistropic growth.